1. Field of the Invention
The present invention relates to a photovoltaic element module and its production method, and a non-contact treatment method, and more specifically to a photovoltaic element module comprising a plurality of electrically connected photovoltaic elements and a method of producing this module.
2. Related Background Art
The problem of the escalating global warming caused by the greenhouse effect, that is, an increase in the amount CO2 in the atmosphere, has produced a growing demand for the development of a clean energy source that does not discharge CO2. One of such energy sources is nuclear power. Nuclear power, however, has many problems, such as radioactive wastes, so a safer clean energy source is desired. Of the expected clean energy sources, solar cells (photovoltaic elements) are gathering much attention due to their cleanness, safety, and easy handling.
At present, the solar cells are roughly classified into a crystal type using single-crystal or polycrystal silicon, an amorphous type using amorphous silicon, and a compound semiconductor type. Of these solar cells, the amorphous type is highly expected. That is, despite its conversion efficiency being lower than that of the crystal type solar cell, the amorphous silicon solar cell has excellent characteristics that are absent from the crystal type solar cell. For example, it can operate in the form of a film because the area of the amorphous type solar cell can be easily increased. Also, it has a large photoabsorption coefficient.
One of the reasons for a slow dissemination of solar cells, despite the attention they have drawn, is their high cost. Various methods have been examined to reduce the production costs of solar cells. The representative approaches include:
(1) Reduction of the production costs of a photoelectric conversion layer,
(2) Efficient utilization of an electric power generating region,
(3) Reduction of the number of connections, reducing connection material and labor costs, and
(4) Reduction of the use amount of covering materials and the material costs.
Of these approaches, the present invention particularly relates to the above point (3). The solar cell connection step is complicated and requires high reliability. However, for simplification and cost reduction, as well as the reduction of the number of parts, an automated mass-production connection method and high speed treatment are required.
FIGS. 11A and 11B are schematic views showing an example of a photovoltaic element which is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-139349 and which has been investigated by the inventors. FIG. 11A is a plan view of the photovoltaic element as seen from its light-receiving surface side, and FIG. 11B is a sectional view of the photovoltaic element shown in FIG. 11A, which is taken along the line 11B—11B in FIG. 11A.
A photovoltaic element 600 shown in FIG. 11, is produced by sequentially stacking a lower electrode layer 603, a semiconductor layer 604, and an upper electrode layer 605 on a conductive substrate 602 of, for example, stainless steel.
The upper electrode layer 605 comprises a transparent conductive film such as of indium oxide or indium tin oxide (ITO) and operates as both a reflection-preventing means and a current-collecting means.
A part of the transparent conductive film is linearly removed at a portion as shown by 601 (an etching line) in FIG. 11A by using screen printing or other methods of applying etching paste containing FeCl3 or AlCl3 to the film and heating it. A part of the transparent electrode film is removed in order to prevent a short circuit from occurring between the substrate 602 and the upper electrode layer 605 when the outer circumference of the photovoltaic element is cut.
In addition, a current-collecting electrode 606 is formed on the surface of the upper electrode layer 605 to efficiently collect generated power. The current-collecting electrode 606 is formed by adhering a metal wire coated with the thin layer of a conductive adhesive (for example, a copper wire coated with a carbon paste) to the upper electrode layer 605 in order to obtain electric power generated in the semiconductor 15 layer without loss. The copper wire is used in order to reduce power loss, and may be replaced by another highly conductive material.
Furthermore, a conductive foil 607 is provided as a further current-collecting electrode in addition to the current-collecting electrode 606. An insulating member 608 is provided under the conductive foil 607 to ensure the insulation provided by the etching line portion, the performance of which cannot be guaranteed.
In the photovoltaic element 600, the metal foil 607 and the substrate 602 function, respectively, as terminals of a positive and a negative electrodes to provide electric power.
It is difficult, however, for this photovoltaic element to be directly used for electric power generation. Since the single photovoltaic element normally generates excessively low power, a plurality of photovoltaic elements must be connected in series or parallel to provide a desired voltage and current. FIG. 11C is a plan view showing an example of series-connected photovoltaic elements (in the case of two series). In this figure, the conductive foil 607 of one photovoltaic element is electrically connected in series to the substrate 602 of another adjacent photovoltaic element by using a connection member 611. Solder is used for the connection and the series connection is completed by carrying out cleaning with a solvent such as MEK (methylethylketone) after soldering.
The conventional method of connecting photovoltaic elements to each other, however, has the following problems.
(1) To fix the conductive foil to the metal substrate by using solder, a part of the substrate must be heated to melt and fix the solder. The heat, however, is transferred through the thermally conductive metal substrate and the semiconductor layer may degrade over a wide area worsening specific characteristics. In addition, defects may occur in the semiconductor layer depending on the heating temperature or time, thereby reducing the yield.
(2) The heat for melting and fixing the solder may degrade the conductive adhesive provided on the semiconductor element to reduce the adhesion strength and electric conductivity, thereby reducing reliability.
(3) Since the solder must be melted, at least about ten seconds are required to heat and cool it, thereby negatively affecting mass-productivity.
(4) If an automatic machine is used for mass production, it is difficult to control the temperature to achieve uniform soldering and to control the tip of a soldering iron. Thus, automation is difficult.
(5) Even when the solvent, such as MEK, is used to wipe off the excess solder, fluxes adhering to the substrate cannot be easily removed to cause rust under high-temperature and high-humidity conditions. Consequently, the covering material of the solar cell may be peeled off.
(6) Using a soldering iron for connections may produce solder residue to reduce the yield. For example, the solder residue may penetrate between the adjacent photovoltaic elements connected in series to cause a short circuit therebetween.
On the other hand, the non-contact treatment method utilizing radiation of laser light, halogen light or electromagnetic waves is widely used in the processing treatment for materials, such as etching, welding or cutting, or in thermal treatment for semiconductor materials. Such a method can more or less solve the problems of the connections using solder.
It is important for the non-contact treatment, however, to efficiently absorb light, heat, or electromagnetic waves. When, for example, a material such as gold, silver, copper, or aluminum is used which are frequently used for electrodes for electric parts, in particular, photovoltaic elements, these materials have a high surface reflectance with respect to the laser light and therefore exhibit a lower efficiency of absorbing energy. Thus, the idea for improving the absorption efficiency of energy is required. Also, a method of improving the absorption efficiency of energy is required that can increase the efficiency of energy absorption without the need for additional steps and that enables stable and high speed treatment.